This invention relates generally to a carburetor fuel mixture adjustment assembly for adjusting the air-fuel ratio of a fuel mixture to be supplied to an engine.
It is known for a carburetor fuel mixture adjustment assembly to include a needle valve body that is threaded into a needle valve receptacle in a carburetor main body. The valve receptacle in such an assembly intersects a fuel passage in the carburetor main body. The needle valve body generally includes an exteriorly threaded portion, a needle and a shank portion disposed between the threaded portion and the needle. The threaded portion of the valve body engages a threaded portion of the needle valve receptacle. The needle of the valve body is positioned within an axially-aligned needle seat orifice of the fuel passage and can be axially advanced and retracted, by rotation of the needle valve body within the needle valve receptacle, to adjust fuel mixture. Axial advancement and retraction of the needle within the needle seat orifice respectively decreases and increases the amount of fuel that can flow through the orifice by decreasing and increasing the cross-sectional area of the valve restriction through the needle seat orifice. The needle valve body is rotated by using a tool such as a screwdriver to engage a screw head of the valve body that protrudes from the carburetor main body. In some such assemblies, to prevent inadvertent or uncommanded rotation of the valve body within the valve body receptacle, a tamper-resistant adjustment needle limiter cap is placed over the screw head and is secured to or braced against an adjacent structure.
Fuel mixture adjustment assemblies of this type include enough clearance between the respective threaded portions of the needle valve body and the valve body receptacle to allow for lateral movement of the needle within the needle seat orifice when force is applied to the valve body head. This lateral movement can change the size of the orifice enough to result in fuel flow rate changes of up to 20% from an optimum fuel flow rate determined by the manufacturer. Fuel flow rate changes caused by needle xe2x80x9cslopxe2x80x9d result in excessively rich or lean fuel mixtures that undesirably increase exhaust emissions. Therefore, it is desirable to reduce fuel flow fluctuations through the needle valve and the resulting increase in exhaust emissions by limiting needle slop.
One example of a stabilizing system for a fuel mixture adjustment needle is disclosed in Japanese Patent Application No. 7-346529 filed Dec. 12, 1995 (Japanese Laid-open Publication No. 9-158783 published Jun. 17, 1997). The Japanese Patent Application describes a carburetor fuel mixture adjustment assembly as described above and including a pressure plate made of an elastic material and overlaid on an outer surface of the carburetor main body. The pressure plate includes an aperture that a protruding portion of the needle valve body must be inserted through during assembly. The presence of the pressure plate limits movement of the needle valve body within the needle valve receptacle by holding the needle valve body in a centered position. The carburetor fuel mixture adjustment assembly disclosed in the Japanese Patent Application also includes an annular sealing member coaxially disposed between the shank portion of the needle valve body and the needle valve receptacle such that the sealing member is compressed between the receptacle and the shank to prevent air from passing between the receptacle and valve body and leaking into the fuel passage. The sealing member is essentially an elongated tube of constant inner and outer diameter that must be forced over a shank portion of the needle valve body then forced into a section of the receptacle shaped to receive the sealing member during assembly. To produce an effective seal against air leakage into the carburetor, machining tolerances must be tight for inner and outer circumferential surfaces of the sealing member, an outer circumferential surface of the shank portion of the valve body, and an inner circumferential surface of the portion of the receptacle receiving the sealing member.
A carburetor fuel mixture adjustment assembly is provided for adjusting the air-fuel ratio of a fuel mixture to be supplied to an engine. The assembly includes an interiorly threaded cylindrical needle valve receptacle formed in a carburetor main body. The valve receptacle intersects a fuel passage formed in the carburetor main body. A needle valve body is supported within the needle valve receptacle. The needle valve body includes an exteriorly threaded portion, a needle and a shank portion disposed between the threaded portion and the needle. The threaded portion of the valve body is in threaded engagement with the needle valve receptacle. The needle is disposable within an axially aligned needle seat orifice portion of the fuel passage. The needle is axially advanceable and retractable by rotation of the needle valve body within the needle valve body receptacle. The needle is movable to positions within the needle seat orifice that respectively decrease and increase the size of a portion of the orifice that is open to fuel flow. The carburetor fuel mixture adjustment assembly also includes a valve spring that is concentrically disposed around the needle valve body between a head of the needle valve body and the carburetor main body. The valve spring is supported in a position biasing the head axially away from the carburetor main body.
The carburetor fuel mixture adjustment assembly also includes a needle stabilizer that engages and cooperates with the valve spring in laterally biasing the needle into one position relative to the needle seat orifice. The lateral bias assures constant fuel flow through the orifice by resisting needle displacement due to such factors as engine vibration and installation of a tamper-resistant limiter cap on an outer end of the needle.
Objects, features and advantages of this invention include a needle stabilizer having a ramped surface that maintains a constant fuel mixture by cooperating with the spring in biasing the needle into the one position, a ramped spring seat that biases the spring against the needle valve body which biases the needle valve body against a side of the valve body receptacle, a spring seat having an easy-to-install horseshoe shape that can be slid between the spring and the carburetor main body, a spring seat integrally formed with a carburetor main body to eliminate an assembly step, a spring that includes the ramped surface, an annular wedge ring that biases the needle valve to a centered position within the valve body receptacle, a sealing member that stabilizes the needle and prevents ambient air from leaking past the needle valve body, a sealing member having an easy to install conical shape that includes independent annular expansion and compression regions that provide a seal between the valve body and valve body receptacle despite rough machining tolerances and any concentricity mismatch between shank and receptacle, and a sealing member compression region that has an outer circumferential contact area greater than an inner contact area of the annular expansion region to insure that the sealing member stays in place when the needle shank is backed out of the seat.